Summary of Work: The potent delta-opioid receptor antagonist family of peptides, consisting of the general formula H-Dmt-Tic-NH-CH(R)-R', tested the length and composition of the linker (R), the position of the carboxylate group relative to Bid ((1H-benzimidazole-2-yl) (R') at the C-terminus of Tic, the effect of N,N-methylation, presence of various amino acids (R?), and a fluorescent moiety. All the compounds exhibited high delta-opioid receptor affinities (Ki < 0.1 nM), while high mu-opioid receptor affinities (Ki < 1 nM) depended two factors: a non-charged C-terminus or a large hydrophobic or aromatic group (e.g., Bid or Ph). Interesting, a Bid-containing analogue exhibited potent delta-opioid receptor agonism coupled to low to very low mu-opioid receptor agonism. The fluorescent derivative proved to be highly selective (greater than 4,000) for the delta-opioid receptor and was a non-competitive or irreversible antagonist; pretreatment of the tissue in vitro or in situ with naltrindole abolished at least 90% of its activity. The data verified that linker chirality is ineffective, a negative charge or N,N-methylation inhibit interaction toward mu-opioid receptors, linker length is critical for the appearance of delta agonist or antagonist activity, and. Dmt is the key residue for all these activities. Synthetic Dmt-containing analogues contained bis-Dmt separated by alkyl chains had high mu-opioid receptor affinities (Ki < 0.1 nM), very good functional bioactivity in isolated tissue assays (Ke = 3-5 nM), and more potent than morphine in vivo; however, analogues containing Dmt and alkyl chains coupled to a pyrazinone ring proved to be five-fold more potent in terms of the presence of analgesia by icv injection. The per oral administration and sc were approximately half as active as morphine. Considering that these molecules are unmodified by lipid or glycosidic constituents, they are quite effective in cross membrane epithelium and passing through the blood-brain barrier.